The present invention relates to an exposure control technique in a semiconductor device manufacturing method and, more particularly, to a control method for an exposure apparatus and a control method for a semiconductor manufacturing apparatus, which can always maintain an optimum process condition in the exposure process.
In the photolithography process in manufacturing a semiconductor integrated circuit, an apparatus called an exposure apparatus for performing pattern exposure is used. An example of the exposure apparatus is a reduction projection exposure apparatus (stepper). In this stepper, light from the light source is transmitted through a mask on which an exposure pattern is drawn so that the pattern is reduced by the optical system and projected onto a wafer.
In pattern formation for transferring a pattern drawn on the mask onto the wafer, the minimum transferable pattern size must be reduced. On the basis of the optical imaging theory, letting NA be the numerical aperture of the projection optical system, and xcex be the exposure wavelength, a resolution (line width) R and depth of focus DOF are given by well-known equation:                     R        =                              k            1                    ⁢                      λ            NA                                              (        1        )                                DOF        =                              k            2                    ⁢                      λ                          NA              2                                                          (        2        )            
where k1 and k2 are process coefficients.
These are called Rayleigh equations and used as criteria to evaluate the imaging performance of a projection exposure apparatus. In response to the demand for shrinkage in feature size of patterns, the exposure wavelength is shortened, the numerical aperture of a projecting lens is increased, and simultaneously, the process is improved. However, since the recent demand for shrinkage in feature size of device patterns is stricter, a sufficient process margin for the exposure amount margin or depth of focus can hardly be obtained, resulting in a decrease in yield.
For photolithography with a small process margin, great importance is being attached to error distribution (error budget) and accurate analysis of error that consumes the process margin. For example, even when a wafer is exposed at the supposedly same set exposure amount to form a number of chips, the effective appropriate exposure amount varies due to PEB (Post Exposure Bake), nonuniformity of development in the wafer surface, or a variation in resist film thickness in the wafer surface, resulting in a decrease in yield. Hence, demand has arisen for an exposure amount and focus control method which effectively uses a small process margin, accurately monitors and feeds back or forward the exposure amount and focus value to prevent a decrease in yield. At the same time, error factors which consume the process margin must be accurately analyzed in units of process units, and major error factors must be removed on the basis of the analysis result.
Two methods have been reported as exposure amount control methods. As the first method, the exposure amount is obtained on the basis of the measurement result of a resist pattern line width or latent image pattern line width. As the second method, the effective exposure amount is obtained on the basis of measurement data of diffraction light intensity that is obtained by irradiating a resist pattern line width or latent image pattern line width with collimated light.
However, the pattern line width changes depending on not only the exposure amount but also the focus value. For this reason, it cannot be determined from the measurement result obtained by the above normal methods whether the line width is affected by a variation in exposure amount value or focus value, or both of them.
On the other hand, two methods have been reported as focus monitor methods. As the first method, the focus value is measured using a variation in size of a monitor mark after exposure due to defocus. As the second method, a variation in focus value is measured as a position shift of a pattern using a mark of phase shift mask type.
In the conventional focus monitor methods, even when the focus shift amount can be obtained and fed back to the focus set value, a variation in appropriate exposure amount value cannot be taken into consideration only using these marks. Hence, the exposure margin cannot be effectively used while suppressing the process variation factors.
The above problems become more serious for finer patterns with a smaller focus margin, or isolated patterns having a smaller feature size. The process variation factors cannot be completely suppressed only by monitoring one of the exposure amount and focus value, and feeding back the obtained measurement result to the set exposure amount, PEB temperature, development time, and the like of the exposure apparatus.
The present invention has been made in consideration of the above situation, and has as its object to provide a control method for an exposure apparatus and control method for a semiconductor manufacturing apparatus, which can suppress a decrease in margin due to process variation factors.
In order to achieve the above object, the present invention has the following arrangement.
(a) According to the present invention, there is provided a control method for an exposure apparatus, in which an exposure amount and a focus value are set in transferring a circuit pattern on a mask onto a resist formed on a wafer by the exposure apparatus, comprising the steps of arranging, on the mask, an exposure amount monitor mark and a focus monitor mark used to separately monitor an effective exposure amount and a focus value on the wafer, transferring the exposure amount monitor mark and the focus monitor mark onto the resist to form an exposure amount monitor pattern and a focus monitor pattern, measuring states of the exposure amount monitor pattern and the focus monitor pattern at least at one of timings after exposure, after post exposure baking, during a cooling process after baking, during a process after cooling, during development, and after development, on the basis of measurement results, calculating a difference between an optimum exposure amount value and an exposure amount set value set in the exposure apparatus and a difference between an optimum focus value and a focus set value set in the exposure apparatus in transferring the exposure amount monitor mark and the focus monitor mark onto the resist, and changing the focus set value and the exposure amount set value of the exposure apparatus in accordance with the calculated differences.
A preferred aspect of the present invention is as follows.
Shapes of the exposure amount monitor pattern and the focus monitor pattern are measured for changes in both of the effective exposure amount and focus shift amount on the resist, which are obtained in advance using the exposure amount monitor mark, and on the basis of measurement results, the difference between the optimum focus value and the focus set value set in the exposure apparatus is calculated in accordance with the shapes of the exposure amount monitor pattern and the focus monitor pattern.
According to the present invention, there is provided a control method for a semiconductor manufacturing apparatus, in which a parameter for photolithography including an exposure process, baking process, cooling process, and development process for transferring a circuit pattern on a mask onto a resist formed on a wafer by an exposure apparatus, comprising the steps of arranging, on the mask, an exposure amount monitor mark and a focus monitor mark used to separately monitor an effective exposure amount and a focus value on the wafer, transferring the exposure amount monitor mark and the focus monitor mark onto the resist to form an exposure amount monitor pattern and a focus monitor pattern, measuring states of the exposure amount monitor pattern and the focus monitor pattern at least at one of timings after the exposure process, after the baking process, during or after the cooling process and during or after the development process, on the basis of measurement results, calculating a difference between an optimum exposure amount value and an exposure amount set value set in the exposure apparatus and a difference between an optimum focus value and a focus set value set in the exposure apparatus in transferring the exposure amount monitor mark and the focus monitor mark onto the resist, and changing at least one of the focus set value of the exposure apparatus, the exposure amount set value of the exposure apparatus, a baking process time in the baking process, a baking process temperature in the baking process, and a development time in the development process in accordance with the calculated differences.
A preferred aspect of the present invention is as follows.
Shapes of the exposure amount monitor pattern and the focus monitor pattern are measured for changes in both of the effective exposure amount and focus shift amount on the resist, which are obtained in advance using the exposure amount monitor mark, and on the basis of measurement results, the difference between the optimum focus value and the focus set value set in the exposure apparatus is calculated in accordance with the shapes of the exposure amount monitor pattern and the focus monitor pattern.
The present invention having the above arrangements has the following functions and effects.
When both the effective exposure amount and the focus shift amount, which are not affected by the focus value, are separately monitored, highly accurate correction can be performed. When exposure conditions are set on the basis of the measurement results, exposure can always be performed with a uniform, maximum exposure margin between lots or wafers or in the wafer surface. For this reason, any decrease in margin or yield due to a process variation can be suppressed.
When the effective exposure amount which is not affected by the focus value is measured using a mark which has a sufficiently large pattern opening and for which a variation in exposure amount can be almost neglected relative to a variation in focus value, or a mark having such a pitch that diffraction light (diffraction light of 1st order or more) at the monitor mark does not enter the pupil of the projecting lens, and only rectilinearly propagating light (0th-order diffraction light) enters the pupil, the pitch of the monitor mark is equal to or smaller than the resolution limit. Hence, flat exposure independently of the focus state can be realized on the wafer surface.
More specifically, assuming an exposure apparatus having an exposure wavelength xcex, a wafer-side numerical aperture NA, a coherence factor "sgr" of illumination, and a magnification M of the mask pattern to the pattern on the wafer, the exposure amount monitor mark is formed by repeating a transparent portion and light-shielding portion at a period p, the transparent portion and light-shielding portion are formed on the same mask at a plurality of ratios, and the period p satisfies                               p          M                ≤                              λ                                          (                                  1                  +                  σ                                )                            ⁢              NA                                .                                    (        3        )            
When both of the effective exposure amount detected using such a mark and the focus shift amount obtained from a focus monitor mark are monitored, and parameters for the exposure process are set on the basis of the measurement results, a decrease in margin due to the process variation factors can be suppressed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.